Carbon baking furnaces, and particularly ring furnaces, are often used in the manufacture of carbon anodes for the aluminum smelting processes. Due to the high temperatures and long baking times, anode baking requires substantial quantities of energy and has become a significant contributor to production cost. Moreover, due to the often relatively low oxygen content in the furnace, pitch is not completely combusted and tends to lead to fires, variations in operating conditions, and maintenance issues for downstream scrubber systems.
Numerous ring furnaces for carbon baking and methods of operating same are known in the art, and exemplary devices and methods are described, for example, in WO 02/099350, U.S. Pat. Nos. 4,215,982, 4,284,404, and 6,339,729, GB 116455, EP 0 541 165, and WO 9855426A1. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
To improve operations, it is known to implement computer control for the firing rates for burners as disclosed in U.S. Pat. No. 6,436,335. However, such control is typically not suitable to remedy incomplete combustion due to lack of oxygen. Moreover, increase of fuel delivery to the furnace will in such systems in all or almost all circumstances not lead to a more complete combustion of volatiles and pitch. As an alternative to burner control, U.S. Pat. No. 4,253,823 teaches use of a water spray/vapor to increase heat transfer between the cooling gas and baked carbon electrodes. While such approach may indeed help reduce fuel demand, other difficulties may arise, including problems associated with downstream condensation and undesirable side-reactions.
In still other known methods, attempts have been undertaken to introduce supplementary air to the preheating zone of a ring furnace to so reduce incomplete combustion of pitch and other undesirable side products as, for example, described in WO 91/19147. Such approach may conceptually be attractive, however, suffers from various drawbacks in practice. For example, where the supplemental air enters the flue in the preheating zone by way of a valve using the negative pressure in the preheating zone flue, the zero point may move in the furnace towards the firing zone. To help overcome this disadvantage, the draft rate at the preheating zone may be increased. However, such increase may adversely affect the temperature gradient in the preheating zone and may not yield desirable heating performance. Still further, the energy gain by introduction of supplemental air for increased combustion will be in most cases neutralized by a temperature drop caused by the addition of supplemental air. Similarly, WO 2004/027332 describes near real-time measurement of soot in the furnace and adjusts the fuel feed rate, draft fan rate, and/or secondary air feed through openings in the zones of the furnace in response to the measured soot level. Notably, the '332 application recognizes the drawbacks of secondary air feed and teaches that secondary air feeds are undesirable and that proper furnace design should eliminate the need for secondary air feeds.
In yet another known attempt to improve energy efficiency, EP 0 158 387 teaches heating of carbon materials in a first pre-heating stage up by use of hot combusted volatile matter, which is obtained by withdrawing the released volatile matter from the first stage, burning the volatile matter outside the first stage, and by recycling the burnt volatile matter to the first stage. Such configuration advantageously improves the pre-heating. Nevertheless, considerable amounts of energy are stilt required for the firing zone of the furnace. Similarly, GB 948038 discloses a baking furnace with a refractory floor and vertical metal flues to so adapt to baking of carbonaceous bodies of widely different sizes and shapes under conditions of increased thermal efficiency, increased unit capacity, reduced furnace construction and operational costs. Among other configurations, the furnace of the '038 reference is configured to allow feeding of the exhaust gas after leaving the furnace back to the combustion source. However, such feedback is typically not suitable for a ring furnace.
Thus, even though numerous configurations and methods for carbon baking furnaces are known in the art, there is still a need for more energy efficient furnaces.